Ignition cable



March 28, 1939.

M. F. PETERS mnmou CABLE Filed June 19, 1936 Me/m'Z/e Z'Pe fans PatentedMar. 28, 1939 UNITED STATES PATENT OFFICE 2 Claims.

(Granted under the act of March 3, 1883, as amended April 30, 1928; 3700. G. 757) This application is a continuation in part of my co-pendingapplication Serial No. 726,895, filed May 22, 1936, now Patent No.2,058,619.

The present invention relates to ignition cables, particularly thosedesigned for transmitting electric energy to spark plugs or otherignition devices of internal combustion engines, and has for its objectthe provision of such a cable which will have a minimum capacitance andamaximum inductance and resistance without subjecting the dielectricmaterial of the cable to excessive electrical stresses, reducing themechanical strength of the cable or causing the spark discharge tobecome aperiodic.

With this object in view the present invention provides an ignitioncable comprising a conductor and insulating means surrounding theconductor, these parts being so designed that the ratio of the diameterof the conductor to the diameter of the cable shall be such that for anyfixed outside diameter of the cable and with respect to the voltagenecessary to fire the plugs, the capacitance of the cable shall have thesmallest value consistent with safe electrical gradient in the insulatoradjacent the conductor, and such ratio shall have the smallest possiblevalue consistent with safe electrical gradient in the insulationadjacent the conductor. In so designing an ignition cable according tothe invention, the mechanical strength of the cable is a furtherlimiting factor in the reduction of the ratio of the diameter of theconductor to the outside diameter of the cable and such ratio isaccordingly reduced only to a value consistent with sumcient mechanicalstrength of the conductor. Where ignition cables according to theinvention are to be employed under conditions of high temperature,certain eifects resulting from the use of heat-resistant insulation mustoften be taken into consideration, and the invention contemplates theprovision of an ignition cable designed to be operable and efiectiveunder such high-temperature conditions.

The invention is designed more in detail in the following specificationwhich is accompanied by a. drawing in which,

Fig. l is a cross-sectional view of a cable which may be constructedaccording to the present invention;

Fig. 2 is a graphic analysis of certain features of the invention, and

Fig, 3 is a schematic view of parts of an ignition system.

It is well known that the ignition system of an internal combustionengine interferes considerably with reception by radio receiving setscarried by vehicles which are driven by such internal combustionengines. This effect is due to the fact that the component parts of theignition system, such as the magneto, leads and spark plugs, act ascondensers and the capacity eflect so produced causes electricalconditions to be set up which react adversely on the signal reception ofany nearby radio receiver.

In order to prevent or decrease the interference with radio receptioncaused by ignition systems, as described hereinbefore, it has become thecommon practice to shield the various parts of the ignition system invarious manners, as by incasing the ignition leads and spark plugs inshields of metal, or other materials. When such shielding harness isemployed it has been found that the capacitance to ground of theshielded ignition system is increased, thereby requiring a greateramount of energy to be supplied by the spark generator to produce therequired breakdown voltage at the spark plugs. While the use ofshielding increases greatly the capacitance to ground of the ignitiohsystem, thereby rendering the present invention particularly valuable inconnection with shielded systems, theinvention is also useful withunshielded systems. In ignition systems employing no shieldingwhatsoever, the problems and effects caused by capacitance are almost asmarked as those in shielded systems. In the shielded systems theshielding harness provides the ground, while in unshielded systems theengine or other metallic part is the ground. The present invention istherefore fully applicable to both shielded and unshielded systems andwhen the capacitance of the ignition system is referred to hereinafter,it is to be understood as referring to the capacitance to ground ofeither a shielded or unshielded ignition system. I

In this connection it has been found that the energy required to bringthe secondary side of an ignition system up to the breakdown voltage ofthe spark-gap may be expressed by the equation H is the energy required,expressed in jouls. C is the capacitance of the secondary side,expressed in farads, and E is the breakdown voltage of the gap expressedin volts.

It will be apparent from this equation that the energy required to bringthe secondary side of the ignition system up to the breakdown voltage ofthe spark-gap varies directly as the capacitance of such secondary side.

It has further been found that in an unshielded ignition systememploying the present type of ignition cable, the capacitance of each ofthe secondary leads is of the order of 40x10- to 110 10- farads. Whenthis same cable is placed in a shielding harness the capacitance of thelongest lead may be increased to 250x10- to 350 10- farads. It will beapparent, there fore, that the increased capacitance due to shieldingcauses a very appreciable increase in the amount of energy required fromthe spark generator to bring the secondary side of the ignition systemup to the breakdown voltage of the spark-gap. This eflect is sopronounced that it has long been observed that magneto and spark coiloperation in shielded systems is not as satisfactory as in unshieldedharnesses.

In studying this problem, I have found that the capacitance of ashielded harness may be reduced by employing a wire in the ignitionsystem which will have such characteristics that a maximum decrease inthe capacitance of the secondary side of the ignition system is obtainedbecause of the use of such wire. Such a wire, in order to obtain theoptimum results should have as low a capacitance per unit length as isconsistent with a good value of electrical gradient for such wire. Inother words, the wire, or ignition cable, should be so designed as tohave the smallest possible capacitance when placed in a metal shieldwithout weakening the cable mechanically or subjecting the dielectric toexcessive electrical stresses.

Referring to Fig. 1 of the drawing it will be seen that a cross-sectionof an ignition cable is disclosed; the same comprising the wireconductor l, the rubber insulation 2, the braid 3, the lacquer I, andthe shielding 5, all of such elements being disposed about the wireconductor in abutting concentric cylinders. It will also be observedthat the elements noted have, respectively, the radii n, 12, 1'3, 1'4and 15. This specific cable constru-ction is shown for purpose ofillustration only, as it will be apparent that a greater or less numberof dielectric and shielding materials may be employedwithout, in anyway, departing from the scope of the invention. In a cable having nlayers of insulation, the outside radius of the cable will berepresented by the term n+1. A schematic view of a shielded ignitionsystem is disclosed in Fig. 3, in which is disclosed the magneto 6,distributor], cable 8, spark plug 9, and shielding III.

In developing the construction of an ignition cable having the bestpractical characteristics as outlined above, I have considered thecomponent members of the cable as a plurality of concentric cylinders.When an electric current is passed through the cable, the lines of forceof the field set up by such current are radial, and the number of linesof force touching the surface of the inner cylinder, or wire, must equalthe number touching the outer cylinder. Since the flux density is equalto the total flux divided by the area, it is obvious that the fluxdensity, and therefore, the gradient, is generally greatest at thesurface of the inner cylinder.

In case a homogeneous dielectric is used and the outer cylindergrounded, the capacitance per unit length may be found as follows:

If the inner cylinder is given a charge #1 per unit length, the electricintensity, or gradient, 2,, at a distance a: from the axis is given bytextbooks as and, since =CE C= centimeters per cm. of length (3) 2 logApplying the conversion factor for transforming capacitance incentimeters into capacitance in farads, this gives r farada per log -;T

V2 0g 1L cm. of length (4) Substituting Equation 3 in Equation 1 anequation for the electric gradient expressed in volts per unit of lengthis obtained, i. e.

dV az= 1: log

volts per centimeter an-H The sign is negative because the innercylinder is at a higher potential than the outer cylinder. In this case,it is the absolute value of the gradient which is of importance and theequation may therefore be written dV E x log The maximum electricgradient occurs where x=ri which is the minimum value of x, or

In case a number of dielectrics are used, as in an ignition cable, thecapacitance and gradient may be determined by assuming each dielectricto represent a condenser and these condensers to be connected in series.The lines of force of flux are radial and are therefore normal to eachboundary surface. The flux density at any distance x from the axis iswhere A is the area per unit length.

The elastance of a number of condensers in series may be written Forconcentric cylinders the capacitance per unit length may be written etc.

ZIOg f I 2 log n in which C1 is the capacitance of a cylindrical tubehaving radii T1 and r: and specific inductive capacitance e1; C2 is thecapacitance of a cylindrical tube having radii r: and Ta and specific inductive capacitance n; and Cn is the capacitance of a cylindrical tubehaving radii Tn and mm specific inductive capacitance :11.

Hence, the total elastance of a series of concentric cylinders may beexpressed as and It the inner cylinder is given a charge #1 per unitlength, the electric intensity at a distance a: from the axis is dx e,e,

and substituting this value in Equation '7, we obtain From the above itwill be seen that mathematical expressions have been developed for thecapacitance and gradient of an ignition cable having any desired numberof dielectrics. These equations or expressions may now be graphicallyrecorded, and in Fig. 2 of the drawing I illustrate such a graphicalrepresentation of the equations.

The curves of Fig. 2 are drawn for the simple case of a homogeneousdielectric as illustrated by Equations 3 and 6. This simplification inno way impairs the usefulness of the curves because the conclusions tobe drawn could also be derived that is, the ratio of the radius of theconductor to the radius of the cable. A second abscissa scale is shown,giving the conductor radius in centimeters when the cable radius isfixed at 0.38 cm. or an outside diameter of approximately threefourthsof a centimeter. The ordinates represent the capacitance per meter ofthe cable expressed in micro-microfarads. It is apparent from this curvethat the capacitance increases from zero for zero radius of theconductor to infinity for a conductor radius equal to the outside radiusof the cable. Practically, neither of these extremes is attainable.

The curve B, illustrated in Fig. 2 is the graphical representation ofEquation 6. a value for E of 15,000 volts and for r: of 0.38 cm. Thevalue for E is the average maximum 'to infinity again for I have usedvoltage I have found necessary to tire the spark plugs found in internalcombustion chambers of airplanes and the value for r: is the averageoutside radius of the ignition cable used with such spark plugs. Theabscissae i'or curve B represent either the ratio or T1 in centimeters,and the ordinates represent the gradient at the surface of the conductorexpressed in ,kilovoits per centimeter. It is apparent from this curvethat the gradient at the surface of the conductor drops from infinityfor a zero radius conductor to a minimum value of approximately 107 kv.per cm. for

equal to unity. If a cable were designed only for minimum gradient inthe dielectric the ratio of the radius 01' the conductor to the radiusof the cable would be chosen for the minimum point of the curve B. Ifthe insulation adjacent to the conductor will safely stand gradients inexcess of. this minimum value a decrease in capacitance may be obtainedby decreasing the ratio It is desired to obtain the maximum possibledecrease in capacitance without subjecting the insulation to excessiveelectrical strains. If the term P is defined as the ratio of thecapacitance for any value of the ratio 1 i E which may be denoted asR,to the capacitance when and the term Q is defined as theratio of thegradient at the surface of the conductor for the same chosen value of Rto the gradient at the surface of the conductor when then the product PQwill represent the relative decrease in capacitance obtained by adecrease of R multiplied by the relative increase in gradient whichmomentarily results when R is decreased. The product PQ has a minimumvalue when R is equal to If the gradient corresponding to this value ofR is in excess of the maximum safe gradient in the insulation adjacentthe conductor, the outside radius of the conductor should be increaseduntil the gradient when R=0.135 is equal to the maximum safe gradient. Ihave found that this is an essential requirement when the capacitance ofthe cable is of importance. If, however, the

gradient when R=0.135 is less than the maximum safe gradient in theinsulation adjacent to the conductor, a further decrease in capacitancemay be obtained by decreasing R until the gradient in the insulationadjacent the conductor is equal to the maximum safe gradient in theinsulation. I have found that in any cable where capacitance is ofimportance, as in cables used in ignition systems of internal combustionengines, the outside radius of the cable should be so related to themaximum safe gradient in the insulation adjacent the conductor as topermit a value for R equal to or less than 0.135.

Another important factor to be considered in building an ignition cableis the gradient at the surface of the cable. This gradient may beeffective in the formation of corona in the air at the surface of thecable, and the corona-formation may have destructive effect on therubber of the cable. The relationship between the gradient in the airspace at the surface of the cable and the properties of the cable may bedetermined from a special form of Equation 9. This form is m=- r log log3 1'1 I:

where n is the radius of the conductor of the cable, 7: the outsideradius of the insulation, and r: the inside radius of the conductingshield.

The specific inductive capacitance of the cable is represented by 21 andof the air, 22. This latter quantity may be considered as unity.Generally r: and r: are so nearly equal that their ratio may beconsidered as unity so that the expression for the gradient at thesurface of the cable becomes r log a The curve C of Fig. 2 is the graphof this equation using values of E, c1 and r: of 15,000 volts, 3.5 and0.38 centimeters, respectively. The abscissae and ordinates are the sameas for curve B except that the ordinate scale is of different magnitude.It will be observed that the form of this curve is similar to the onefor capacitance.

In most high voltage cable design, the choice of the diameter of theconductor is of first importance. This value is chosen with regard tothe permissable resistance of the cable per unit length. I have foundthat the resistance of the conductor in an ignition cable is not animportant factor, as regards its effectiveness in transferring energyfrom a spark coil or magneto to a spark plug, consequently norestrictions in design need be imposedby resistance considerations ofthe conductor.

In fact, I have found that from the standpoint of reduced radiointerference and efficient spark plug operation, a high resistance isdesirable. This point of view is opposite to that of present practice inignition cable design. The upper limit to the resistance occurs when thetime constant of the circuit becomes appreciable or the energy lost inthe conductor becomes an appreciable part of total energy transferred.Using present known alloys or metals this upper resistance limit wouldnot be reached unless the conductor diameter were reduced to a point farbeyond the limit permitted by safe electrical gradient and mechanicalstrength. This means that in the development of an ignition cable ofminimum capacitance according to the present invention no limitations asto conductor size or material will be imposed by resistancerequirements.

I have therefore found that the ignition cable of minimum capacitance isthe cable with the conductor of smallest diameter consistent with safegradient in the dielectric and suflicient mechanical strength. I havefound from Equation 6 that the optimum ratio of the radius of theconductor of this wire to radius of the cable is given by the followingequation -s R =Q" (12) Where R-fl E is the magnitude of the highestvoltage necessary to fire the spark plug, G is the magnitude of themaximum gradient the dielectric of the cable 7 can stand withoutbreakdown and r: is the outside radius of the cable. Practically, thevalue of R may be determined from curve B of Fig. 2. The value of R willbe the abscissa of the point on the curve whose ordinate is equal to G.I have found that if rubber is used as the dielectric. having a valuefor G of about 240 kv. per cm. the values for R and n are approximately0.06 and 0.023 cm. respectively, or a conductor diameter ofapproximately one-half millimeter. I have found that a large number ofthe rubber insulating compounds of ignition cables now in use withstandgradients of this magnitude. I have also found that if a wire of thissize be made of steel, stainless steel, phosphor bronze or othermaterial of high tensile strength, its mechanical properties aresatisfactory. I have also found that the use of such high tensilestrength conductors instead of the customary copper offers additionaladvantages over copper because of their higher resistance. The higherresistance of the ignition cable reduces radio interference because ofincreased damping in the interfering waves and reduces the intensity ofthe current in the spark causing less wear on the spark plug electrodes.

The previous equations and curves refer to a cable with a solidconductor. In the practical application of the art stranded conductorsare generally used in the construction of ignition cables. Thisdeparture will not materially affect the capacitance consideration butit will modify the gradient at the surface of the conductor.

From a practical standpoint only the change in the gradient at thesurface of the conductor need be considered when the change is made froma solid to a stranded conductor. This correction to the gradientequation may be adequately made by use of a constant Kn which representsthe per cent increase in the gradient in the dielectric at the surfaceof the conductor when a change is made from a solid conductor to astranded conductor of n strands and having the same total crosssectional area as the solid conductor. The value of Kn may be consideredas a constant as regards both the range of ratio of radii of conductorto cable and the number of strands in a stranded cable which are ofimportance in ignition cable design. The equation which defines thedesired ratio of conductor radius to cable radius may then be written RRrzG value for G of 240 kilovolts per cm. is used, the value of G/ (1+Kn)is about 180 kilovolts per cm. and the abscissa corresponding to thisordinate is 0.086 such abscissa being chosen from a point on the curveto the left of the minimum thereof. This corresponds to a radius for theconductor of 0.032 cm., or a diameter of approximately threefifthsmillimeter. I

A further advantagearising from this use of the smaller conductor isthat the gradient at the outside surface of the cable will be less,since as shown in curve C of Fig. 2, this gradient decreases as thediameter of the conductor decreases. Therefore the possibility of adetrimental effect on the rubber dielectric from corona is less becauseof the use of this wire.

The definition of the optimum value for the ratio of the radius ofconductor to radius of the cable will be modified further by thepresence of other dielectrics in the cable such as braid and lacquer asshown in Fig. 1. The general equation for the magnitude of the gradientat the surface of the conductor in the general case of a number ofdielectrics may be obtained from Equation 9 and is i. e., the ratio ofthe conductor radius to the outside radius of the cable.

The value of optimum radius in the general case will then be (1+K;. nmG

Where M is a constant defined as follows in mH) The particular conductorsizes set forth hereinbefore are entirely practicable when a goodinsulating material is employed and when it is not necessary to takeinto consideration thermal conditions under which the ignition cable maybe used. Where, however, the ignition cable is used under hightemperature conditions, the primary consideration is the provision of aheat resistant insulating compound, and it has been found thatinsulating compounds having good thermal properties do not, usually,have good electrical properties. Consequently, if an insulator havinggood thermal properties and poorer electrical properties must be used,it becomes necessary to reduce the maximum allowable gradient at thesurface of the conductor, in order to prevent failure of the cable. Inorder to bring the gradient within allowable limits the radius of theconductor may be increased, and I have found that a conductor made up ofseven strands of stainless steel wire, each strand having a diameter ofapproximately .013 inch, provides satisfactory results under theconditions set forth. Such a stranded conductor has a diameter ofapproximately .039 inch (approximately .99 millimeter) or a diameter ofapproximately one millimeter. The corresponding solid conductor has adiameter of approximately .0172 inch (approximately .872 millimeter) ora diameter of approximately seven-eights of a millimeter. In neithercase need the outside radius of the cable be changed.

The conductor dimension values given hereinbefore in the several casesare not the only conductor or cable radii which fall within the scope ofthe invention, and such values of conductor and cable radius are to beconstrued as illustrative of constructions coming within the broad scopeof the invention, and as illustrating the fundamental theory on whichthe invention is predicated, namely that the capacitance of the cable isto be made as small as possible, and the resistance is to be increased,all without increasing the electrical gradient above allowable limitsand without decreasing the mechanical strength of the cable belowallowable limits.

It will be apparent to those skilled in the art that while I havedisclosed my invention with particular reference to an ignition cableconstruction, the invention is not limited to the field of ignitioncables, but may be employed in designing and constructing an electricalconductor for use under any conditions where low capacitance or highresistance of the conductor is to be achieved. Obviously, cablesconstructed according to my invention may ,be employed for anyelectrical use without departing from the spirit or scope of theinvention, and the invention is not to be considered as being limited toignition cable construction.

It is to be expressly understood that the present invention is notlimited to any specific modification disclosed herein, or otherwise thanby the appended claims.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

I claim:

1. An electrical cable comprising a conductor and insulating meanssurrounding the conductor, the diameter of the conductor being sorelated to the outside diameter of the cable that the relationshipPQ;0.135 is obtained, in which P is the ratio of the capacitance of thecable to, the capacitance when the ratio of the radius of the conductorto the radius of the cable is 2. An electrical cable comprising aconductor and dielectric means surrounding the conductor, the cablebeing so constructed that the ratio of the radius of the conductor, '1,to the radius of the dielectric means, '2, is equal to I. 6' in which eis the natural logarithmic base.

mm 1'. mm.

